1. Field of the Invention
The present invention pertains to the field of radiation measurement within the human body. More particularly, the present invention pertains to the field of in-vivo dosimetry using a single scintillating fiber inserted into the human body for accurate radiation measurement.
2. Description of the Related Art
Radiation is often used in the treatment of human health problems. These problems, generally occurring “in-vivo”, include cancer and, more recently, the re-closing of arteries after balloon angioplasty. Knowledge of radiation levels within the body, important for the success of such treatment, is usually pursued by calculation or modeling because direct measurement of internal body radiation is often too difficult to carry out.
In the field of radiation measurement within the human body, the following related art documents are known (all of which are incorporated herein by reference): U.S. Pat. No. 4,932,412 entitled “Intraoperative and Endoscopic Tumor Detection and Therapy”; U.S. Nuclear Regulatory Commission Report NUREG/CR-5223 entitled “Scintillating Fiber Detector for In-Vivo Endoscopic Internal Dosimetry”, published October 1988; Phys. Med. Biol., 1992, Vol. 37 No. 10, pp. 1883–1900 entitled “Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: I. Physical characteristics and theoretical considerations”; U.S. Pat. No. 5,704,890 entitled “Real Time Sensor for Therapeutic Radiation Delivery”; Japanese Published Unexamined Patent Application 10-213663 entitled “Local Dosimeter”; U.S. Pat. No. 5,880,475 entitled “Scintillation Fiber Type Radiation Detector”; U.S. Pat. No. 5,905,263 entitled “Depth Dose Measuring Device”; U.S. Pat. No. 6,151,769 entitled “Method of Making a Scintillator Waveguide”; and Japanese Published Unexamined Patent Application 2001-56381 entitled “Local Radiation Amount Measuring Device and Medical Device Equipped Therewith”.
The systems noted above require special corrections or fiber assemblies to remove errors arising from Cerenkov radiation, which generally complicates the use of scintillating fibers for therapeutic radiation. Cerenkov radiation is the radiation that results when a charged particle, moving in some medium, travels faster than light does in that medium. (The charged particle can be introduced ‘on its own’, or can be an electron kicked out of an atom by an entering photon=gamma ray or X-ray.) The speed of light in a medium is given by its index of refraction. The speed of a particle is given by its mass and its energy. The index of refraction in most scintillating fiber materials is 1.6. Thus, an electron with energy more than about 0.14 MeV introduced on its own, or kicked out of an atom by an entering photon (=gamma ray) of energy more than about 0.28 MeV, will be travelling fast enough to generate Cerenkov radiation. Most High Dose Rate radioisotopes (e.g. Ir-192 and P-32, which are introduced into the body via a catheter or the like during HDR afterloader therapy), and all ‘external X-Ray beam’ sources, will have enough energy to trigger Cerenkov radiation. Low Dose Rate radioisotopes (e.g. I-125, Pd103) used in medical implant seed therapy may not emit anything with enough energy to trigger Cerenkov radiation.
Cerenkov radiation can occur in most scintillating fibers. In the prior art, there are two common ways to remove the errors arising from Cerenkov radiation. The first is to employ special filter assemblies within the measuring fiber to remove the Cerenkov light component (see e.g. FIG. 1 in JP 2001-56381 A). The second is to include an additional reference (or background) fiber, without a scintillating tip, which is exposed to the ambient radiation (see FIG. 1 in Phys. Med. Biol., 1992, Vol. 37 No. 10, at page 1886). The reference fiber also produces the Cerenkov light component which can then be subtracted from the output signal of the measuring fiber to produce a corrected signal substantially free of the Cerenkov light component.
A third way to remove the Cerenkov light component is mathematically. This way is preferred since physical modifications to the fiber assembly are not required and it can therefore lead to a simpler and cheaper way to perform in-vivo radiation dosimetry. However, previous attempts to mathematically model or predict the Cerenkov radiation component have required complex software to evaluate complex mathematical formulas and have also required a knowledge of the geometrical shape of the fiber path (see FIGS. 12 and 13, and sections [0062] to [0075], of JP 2001-56381 A).